For the production of absorbent articles, such as toilet paper, paper towels, paper handkerchiefs or the like, a cellulose material, i.e. made of cellulose fibers, is used as a base material in the form of one or more plies bonded together and variously decorated or worked to achieve particular aesthetic and technical-functional features. The cellulose material used for this type of production is a paper with special softness and absorption capacity features and is usually called “tissue” paper.
One of the most frequently used processes for converting tissue paper for obtaining products of the type mentioned above is embossing. This operation consists in passing the cellulose material in a nip between two rollers provided with protuberances and/or cavities and optionally one of the two provided with an elastically yielding surface, for imparting a permanent mechanical deformation to the base cellulose material. This permanent mechanical deformation, which involves a dislocation, a deformation or a breakage of the fibers of the base material, allows increasing the apparent thickness of the cellulose material and imparting particular technical-functional features to the latter, such as for example a greater apparent thickness, greater absorption capacity, better tactile features, a surface treatment adapted to increase the ability of the material to remove solid or liquid materials from a surface, and other features known to those skilled in the art.
Embossing is also used to impart a particular aesthetic feature to the product, i.e. to make decorations thereon formed by cavities in the outer surface of the cellulose material, corresponding to protuberances of the cellulose material facing towards the interior of the material itself, when this is formed by at least two or more plies bonded together.
Embossing is carried out in embossing units or embossing-laminating units. These latter also have the function of mutually bonding two or more plies, of which at least one is embossed or in which both are embossed separately from each other. Bonding takes place by applying a glue on at least some of the protuberances generated by embossing on at least one of the plies forming the multilayer material using a glue applicator usually arranged along the circumferential development of an embossing roller, upstream of a lamination nip or a gluing nip at which two or more plies are glued together. Lamination of the plies, with mutual compression of one ply against the other to facilitate and promote adhesion, can take place in the gluing nip, usually defined between two embossing rollers, or downstream thereof by using a laminating roller which cooperates with one of the embossing rollers.
For example, U.S. Pat. No. 3,961,119 describes a material having two plies, which are embossed separately with helical patterns at different inclinations and bonded at the intersection of the helical patterns.
U.S. Pat. No. 6,470,945 describes a machine for embossing plies which, once bonded together, form a web material with plies coupled tip-to-tip or nested, respectively. Tip-to-tip bonding means a bonding in which the protuberances of one ply are located at least partly at protuberances of the other ply. Nested bonding, conversely, means a bonding in which the protuberances of one ply nest between the protuberances of the other.
Other embossed multi-ply products and respective devices for the production thereof are described and illustrated for example in U.S. Pat. Nos. 6,261,666, 6,109,326, US-A-2003/0102096, U.S. Pat. No. 6,136,413, WO-A-00/78533, U.S. Pat. Nos. 6,681,826, 3,414,459, 5,173,351, 6,032,712, 6,245,414.
U.S. Pat. No. 4,320,162 discloses a multilayer two-ply product embossed with equal embossing on the two sides, i.e. on the two plies. The embossing is arranged tip-to-tip and consists of large protuberances with a large front surface, between which protuberances with smaller height and smaller size are positioned.
WO96/18771 discloses a multi-ply cellulose product in which the embossed ply has a background micro-embossing and a decorative embossing formed by protuberances with a linear development, i.e. with a longitudinal dimension substantially larger than the transverse dimension. The protuberances forming the decorative embossing are arranged in areas devoid of micro-embossing. The production of this material is obtained by an embossing roller which has a first series of micro-protuberances having a first height, and macro-protuberances of larger size and greater height, provided in areas devoid of micro-protuberances. The micro-protuberances form the background embossing of the cellulose material, while the macro-protuberances form the decorative embossing.
WO99/44814 discloses a method and a device for embossing a multi-ply cellulose material, in which a ply of cellulose material is sequentially embossed twice, using two separate embossing rollers, to obtain a combination of background and decorative protuberances.
According to another production technique, called TAD (Through Air Dryer), a cellulose material having a large volume and a high thickness, accompanied by a high softness, is obtained by drying the cellulose ply with hot air flows, which increase the volume thereof, on forming nets or fabrics, the structure of which is copied on the outer surface of the ply, forming a sort of embossing. TAD plants are extremely expensive and require large amounts of energy.
There is a constant research to identify techniques for embossing, laminating and bonding plies which allow optimizing the yield of these materials from many, even conflicting points of view.